1. Field of the Invention
The present invention relates to a disk bonding apparatus and a disk bonding method, and more particularly to the manufacture of an optical information recording medium obtained by bonding two substrates.
2. Description of the Related Art
A technique for reproducing or recording information having a high density by utilizing a laser beam has been known and has mainly been put into practical use as an optical disk. The optical disk is roughly divided into a read-only type, a write-once type and a rewrite type. The read-only type has been merchandized as a disk referred to as a compact disk recording music information or a disk referred to as a laser disk recording image information. Then, the write-once type has been merchandized as a document file or a static image file. Furthermore, the rewrite type has been merchandized as a data file for a personal computer.
In general, the optical disk has such a configuration that an information layer is provided on one of the main surfaces of a transparent resin substrate having a thickness of 1.2 mm and a protective film such as an overcoat is provided thereon or a protective plate which is identical to the substrate is bonded with a radiation curing resin.
In recent years, various investigations have been made to use an objective lens having a small laser wavelength and a great numerical aperture (NA) in order to increase the density of the optical disk. However, a reduction in the wavelength and an increase in the NA decrease an allowable value of a tilt angle (tilt) of the disk in a direction of irradiation of a laser beam. In order to increase the allowable value of the tilt, it is effective that the thickness of a substrate is reduced and the substrate of a digital video disk (DVD) has a thickness of 0.6 mm, for example.
If a resin substrate having a thickness of 0.6 mm is used singly, the substrate has an inferior mechanical strength. Therefore, two substrates are bonded with an information recording surface provided inside.
For the bonding, a main bonding method includes a method of applying a radiation curing resin onto a substrate and closely bonding another substrate to irradiate radioactive rays, thereby carrying out curing (which will be hereinafter referred to as a radiation curing method). For the radioactive rays, ultraviolet (UV) rays are generally used.
In such a radiation curing method, generally, a radiation curing resin is applied like a doughnut while rotating a substrate at a low speed and a substrate to be bonded is superposed thereon so that two substrates are integrated. Then, a rotation is carried out at a high speed and the radiation curing resin is fully diffused between the substrates and is thereafter cured by the radioactive rays.
In a radiation curing method in both directions of a substrate, for example, Unexamined Patent Publication Hei.09-128823 discloses a curing method and a curing apparatus. In this method and apparatus, an optical disk is inserted between UV curing resins and is applied UV rays from the outer side of the UV curing materials. Unexamined Patent Publication 2001-216692 also discloses a curing apparatus by irradiation form both directions of a disk. In this apparatus, each of amounts of irradiation from both directions is controllable respectively during curing. In the above mentioned techniques, conventionally, both radioactive ray irradiating operations are started at the same time.
Moreover, a warpage test in an apparatus is also carried out. A result of measurement obtained by the warpage test is used for only discrimination of excellent and inferior products.
The curing (gelation) of a bonding material (radiation curing resin) between substrates proceeds instantly (within 0.2 second) from an interface of both substrates with the radiation curing resin. The curing also progresses in a fixed speed. In a curing apparatus having such a structure, it is hard to adjust a curing degree balance in a radiation curing resin layer (a bonded layer) based on a radiation power or a radioactive ray irradiation time so that it is difficult to control the warpage. In short, it is hard to control the warpage of a disk for the conventional curing apparatus which can control only a term of irradiation and amount of irradiation.
Moreover, the result of the measurement obtained by the warpage test in the apparatus is used for only the discrimination of the excellent and inferior products. Therefore, there is a problem in that the adjustment for decreasing the warpage is to be carried out manually.
The invention has been made in consideration of the problem and has an object to provide an optical recording medium (disk) bonding apparatus which eliminates the warpage of a disk during bonding and has high precision in the bonding.
The disk bonding apparatus according to the invention is characterized by means for bonding two substrates and then irradiating radioactive rays from both sides of the substrates when carrying out radiation curing and a mechanism for shifting irradiation starts of both radioactive rays, thereby controlling an amount of warpage of each substrate after curing.
More specifically, a first aspect of the invention is directed to a disk bonding apparatus comprising first and second irradiating means provided with a first substrate and a second substrate to be opposed to each other, having the first and second substrates superposed with a radiation curing resin between the two substrates and serving to irradiate radioactive rays from both sides thereof, the radiation curing resin being cured to bond the first and second substrates, wherein irradiation timings of the first and second irradiating means can be adjusted.
This method utilizes that the radiation curing resin is provided between the two substrates and the substrates and the curing resin are integrated by a diffusion generated through a rotation. Then, radioactive rays are to be irradiated from both upper and lower surfaces of the integrated substrates. When this process of the irradiation, upper and lower irradiation start timings are intentionally shifted. The progress for a curing degree in a bonding material layer is started from the interface of the radiation curing resin with the respective substrates. Moreover, the curing progresses in a fixed speed. This method utilizes the property of the curing, it is possible to change a curing balance in a radiation curing resin layer by the intentionally shifting the upper and lower irradiation start timings. Therefore, it is possible to efficiently adjust a warpage state after the curing.
In a second embodiment it is desirable that the disk bonding apparatus should comprise a warpage measuring mechanism for measuring warpage after bonding and curing, and a timing control means for suppressing the warpage by shifting the irradiation timings of the first and second irradiating means based on a result of the measurement of the warpage measuring mechanism. This embodiment can be utilized where the timings of the first and second irradiation timings are adjusted based on the measurement of the warpage after curing to improve the warpage of subsequent disks.
By such a structure, automated control can be carried out and the bonding can be performed efficiently with small warpage and high precision without requiring an additional device.
A second aspect of the invention is directed to a method of bonding a disk comprising the steps of superposing a first substrate and a second substrate with a radiation curing resin between the two substrates and bonding the two substrates to irradiate radioactive rays from both sides thereof, wherein the irradiating step includes a step of controlling an amount of warpage of each of the substrates after curing by shifting irradiation starts of both radioactive rays such that the irradiation start of the radioactive rays is carried out earlier on a substrate side in the first or second substrate where a direction of the warpage is to be predominant.
It is desirable, in the second embodiment, that the method of bonding a disk should further comprise the steps of measuring warpage of the disk after the irradiating step and adjusting an irradiation timing at the irradiating step such that the warpage of the disk is suppressed based on a result of the measurement to improve the warpage of subsequent disks.
According to the method, automated control can be carried out and the bonding can be performed efficiently with small warpage and high precision without requiring an additional device.